The present invention relates to a headphone having earpieces which each comprise at least one electroacoustic transducer operating on the isodynamic principle and including a small-mass diaphragm at least corresponding in size to an average auricle, and in which headphone the coupling space of each of the earpieces is designed for minimum sound reflection.
Headphones of the isodynamic type are equipped with an ear cushion which, with the headphone in use, closes the coupling space tightly to the outside. In headphones with moving-coil systems however, the so-called open design is also usual in addition to those using a tight enclosure above the ears. If a headphone with the dynamic transducer systems and diaphragms having minimum masses is operated with a tightly closed coupling space, the restoring force of the air enclosed in the coupling space causes the diaphragm resonance to rise to between 1,500 to 4,000 Hz, regardless of the fundamental resonance of the diaphragm which may range between 70 and 250 Hz even with attachment on all sides. Such diaphragms, for example, use a polyester foil having a thickness of 6 microns and are provided with a 6 micron thick conducting coating of aluminum extending over the entire surface area thereof, with the surface area of the diaphragm corresponding at least to the extension of an average auricle. This means that in the range between 0 and 1,500 or 4,000 Hz, the diaphragm is elastically impeded and oscillates with a constant amplitude if the driving force is constant and independent of the frequency. This results in a constant pressure in the closed coupling space. Above this critical resonance frequency, the amplitude would decrease with frequency, however the diaphragm, due to its extension, emits directional sound waves, which again result in a linear response of the headphone. A linear response therefore implies the requirement of a very small mass of the diaphragm, a very low fundamental resonance of the diaphragm, and a strong restoring force in the tightly enclosed coupling space. It is noted that elastic impedance is caused by an air cushion in the coupling space which acts like a spring against the diaphragm.
In practice, however, some difficulties arise in the use of headphones of the above described kind and with a tight closure of the coupling space. These difficulties hinder high-quality sound transmission and can be overcome only in an expensive way. The difficulties particularly involve acoustic disturbances in the coupling space caused by the sealing ear cushion whose inside surfaces can hardly be made non-reflecting, which has the effect that the acoustic event is localized "close to the ear" or "in the ear". Compensating means such as acoustic frictional resistances built into the frame surrounding the ear are very costly.